It's like the backside of your worst nightmares. It's all blood and bone and darkness. Filled with the bodies and souls of all things hungry, sharp, and nasty.
Purgatory is a realm where the souls of supernatural creatures go after they die. Its physical location is separate from both Heaven and Hell. Crowley estimates there are 30-40 million souls there. It was originally created by God to contain the oldest creatures, Leviathans, for fear they would destroy the rest of creation.
Crowley makes a deal with Castiel that they will locate Purgatory in order to access all the souls there in order to use the power of the souls housed there. They instigate a plan where they use Samuel Campbell, getting his agreement with a promise to resurrect Mary, to capture the Alpha monsters who they are convinced know its location.
When Dragons raise Eve, the progenitor of all monsters who has been in Purgatory for 10,000 years, Castiel and Crowley know that capturing her can give them access to Purgatory but the plan is thwarted when Dean kills her.
Eleanor Visyak, an old friend of Bobby's, is revealed to be a creature that escaped Purgatory in 1937. Castiel and Crowley torture her to death in order to obtain her blood and the secret ritual which will open a door to Purgatory. After Castiel opens the portal, he imbibes millions of souls and proclaims himself a god.
Desperate to stop Castiel's wrathful rampage on Earth, Sam, Dean and Bobby bind Death in order to Kill Castiel. Castiel releases Death, who tells them Castiel has taken in not only the souls of monsters, but the Leviathans. He tells them that they need to re-open the door to Purgatory and return the souls and creatures. He arranges for another lunar eclipse so they can perform the ritual. Dean opens the door to Purgatory and the souls do return, but the Leviathans are too strong, and stay on earth inside Castiel. Later, Dean, with the help of Castiel, kills Leviathan leader Dick Roman which results in him and Castiel getting sucked into Purgatory.
Castiel disappears after and Dean tries to find him while battling the monsters who live there. A vampire named Benny tells Dean there is a way a human can escape Purgatory, and he'll help Dean find it for Dean taking him through the portal. Dean escapes Purgatory with Benny's soul inside him, allowing him to resurrect the vampire. Eventually angels, under the command of Naomi, invade Purgatory and rescue Castiel at the cost of many angels lives.
When Sam needs to travel to Hell to rescue Bobby Singer's soul as part of the second trial to close the gates of Hell, rogue Reaper Ajay takes him to Purgatory where there is a "backdoor" to Hell, a portal located behind a boulder between three trees that grow together. Sam is able to use the portal to travel to Hell and back with Bobby, but they end up trapped in Purgatory when Ajay is killed by Crowley. However, Benny returns to Purgatory and leads them to the exit portal, but stays behind to hold off a group of vampires rather than escaping again.
3. Crowley and Castiel torture Eleanor Visyak and find a ritual to open a door to Purgatory. Castiel uses the ritual, which involves drawing a series of sigils in a mixture of the blood of a virgin and the blood of a Purgatory native. A spell is then recited on the night of the lunar eclipse:
Ianua Magna Purgatorii Great door of Purgatory
Clausa Est Ob Nos That is closed for us
Lumine Eius Ab Oculis Its light is kept away
Nostris Retento From our eyes
Sed Nunc Stamus Ad Limen Huius But now we're standing on this threshold
Ianuae Magnae Et Demisse Of the Great door, and humbly
Fideliter Perhonorifice Faithfully, respectfully
Paramus Aperire Eam Are preparing for its opening
Creaturae Terrificae Quarum Ungulae Terrible creature, whose claws
Et Dentes Nunquam Tetigerunt And teeth have never touched
Carnem Humanam Aperit Fauces Human flesh, is opening its jaws
Eius Ad Mundum Nostrum Nunc Toward our world now
Ianua Magna Great door
Aperta Tandem! Open at last!
4. After Death agrees to engineer another lunar eclipse, Dean uses the same ritual and blood to reopen a portal to Purgatory, allowing all the monstrous souls inside Castiel to return - except those of the Leviathans. 
6. Apparently in case humans ever accidentally got transported there, God put in a portal that humans could use to escape, but no other monster could use. Benny leads Dean and Castiel to the location of the portal which someone had told him and it opens in reaction to Dean's presence. Carrying Benny's soul in his arm, Dean is able to make it through just before it closes, allowing him to later resurrect the vampire, but Castiel purposely lets go, wanting to stay. It is unclear if an angel would have made it through the portal anyway. Sam is later able to use this portal to escape with the soul of Bobby Singer after briefly getting trapped in Purgatory himself.
9. Along with the portal to Earth, a hidden portal exists to Hell that acts as a "backdoor" into the realm, allowing beings to sneak in and out of Hell. Rogue Reapers use this to transport mortals and souls between Earth and Hell for a price and Sam uses it to rescue Bobby Singer. 
After resurrecting Samuel Campbell, Crowley has him tracking down high-ranking creatures, especially Alphas, so that he can question them about the location of Purgatory. Crowley says he is interested in Purgatory for "development purposes."
- Crowley: Location, location, location. I'm a developer. Purgatory is vast, underutilized, and hell adjacent, and I want it.
Crowley tortures and interrogates the Alpha Shapeshifter for Purgatory's exact coordinates. The Alpha refuses to reveal its location, claiming ignorance, so Crowley angrily retaliates by decapitating it with a sword. Later in the episode, Crowley appears to be killed when Castiel burns a pile of old bones.
Death encourages Dean to continue his and Sam's current investigation. He is most likely referring to their investigation into Crowley's pursuit of Purgatory. He also tells him that "it's about the souls."
- Death: You throw away your life because you've come to assume that it will bounce right back into your lap. The human soul is not a rubber ball. It's vulnerable, impermanent, but stronger than you know and more valuable than you can imagine.
- Right now you're digging at something. Intrepid detective. I want you to keep digging, Dean. It's about the souls.
Purgatory is revealed not only to be the place where supernatural beings go after death, but also to be the place where the "Mother of All" is located. A group of Dragons are collecting virgins in an attempt to raise the Mother from Purgatory and allow her to walk the Earth once again. Sam and Dean discover an ancient manuscript made of human skin that Bobby ultimately deduces is a manual for accessing Purgatory and bringing forth Eve.
Toward the end of the episode, this plan is carried out by the remaining dragons, who use the knowledge gained from the manuscript to open a door to Purgatory. They take one of their captives and throw her into the fiery chasm below. Out of the portal, the girl rises again as Eve.
Eve tells Sam and Dean that Crowley is alive and still torturing her children to find Purgatory.
In a flashback, Crowley convinces Castiel that if they find Purgatory they can split the souls inside, so that Castiel can defeat Raphael and Crowley can secure his position as the leader of Hell. In the present, Crowley is dissecting Eve for clues that might lead them to Purgatory, but to no avail. He insists that if Eve were alive, and not killed by the Winchesters, she could have led them there.
Bobby reveals that there was another person who attempted to open the door to Purgatory: a horror writer named H.P. Lovecraft. In 1937 in Providence, Rhode Island, Lovecraft held a dinner party with five other people, and they performed a ritual to open a portal that led to Purgatory. Eleanor Visyak emerged through the door as an invisible force that possessed Lovecraft's maid. She then may have proceeded to kill all of the ritual participants over the next year, though she denies this and the person shown in the killing scene of H.P. Lovecraft does not fit her description (though it was shadowed).
Castiel and Crowley continue their attempt to access the souls in Purgatory. Having captured Eleanor Visyak, they torture her until she reveals the ritual and what they need for it. They must draw a sigil using a mixture of virgin blood and the blood of a "Purgatory native" and say the ritual words on the night of the lunar eclipse. They use the blood of Eleanor, and then Crowley adds the virgin blood and gives the mixture to Castiel. Castiel decides to deny Crowley his share of the souls, giving Crowley the option to "flee or die." Crowley allies himself with Raphael in retaliation, and the two force Castiel to leave while they conduct the ritual themselves. Castiel deceives them by switching the correct blood mixture with dog blood and performing the ritual by himself. He then proceeds to absorb the souls of Purgatory all at once and destroys Raphael after Crowley teleports away. Sam tries to kill Castiel with an Angel Killing Sword, but Castiel proves immune. He then declares himself to be the new God.
Death tells Sam, Dean and Bobby that in order to control Castiel, they need to re-open the door to Purgatory and return the souls. He arranges for another lunar eclipse so they can perform the ritual. Dean and Bobby open the door to Purgatory and help Castiel return the souls, however The Leviathans are too strong, and stay on earth inside Castiel taking full control of his Vessel.
Every soul here is a monster. This is where they come to prey upon each other for all eternity.
Dean and Castiel succeed in slaying the head Leviathan, Dick Roman, with the righteous bone soaked in the blood of the fallen: a fallen angel, the ruler of fallen humanity, and the father of fallen beasts. Upon death, Dick explodes so violently that Dean and Castiel are pulled into Purgatory along with him. They awake to find themselves in a dark wooded area, with shadows and shapes moving in the trees. Dean asks how they can escape, to which Castiel replies they'll likely be torn to pieces instead. Castiel then disappears leaving Dean alone surrounded by the souls of monsters.
Some time after being separated from Castiel, Dean, armed with a makeshift blade he made in Purgatory, interrogates a vampire for Castiel's location and decapitates him when he doesn't get a response. He is attacked by another vampire, but is saved by the vampire Benny who decapitates Dean's attacker and reveals that God put in a way out for humans that get trapped in Purgatory. Benny offers Dean a deal: he'll lead Dean to the portal if Dean allows his soul to hitch a ride out with him when he goes and warns him never to trust anyone in Purgatory.
A year after being trapped, Dean escapes Purgatory with Benny's soul, ending up in the 100-mile Wilderness in Maine. Dean resurrects Benny and later tells Sam that Castiel died in Purgatory, that it got hairy near the end and Castiel "let go," but doesn't elaborate on how he escaped. During a later conversation with Dean, Benny agrees with Dean that Purgatory is "pure" and wishes he appreciated that fact like Dean did.
In a flashback, Dean and Benny interrogate a monster on the location of Castiel and after getting it, Dean kills the monster with Ruby's Knife. The two find Castiel who admits he abandoned Dean as he knew many powerful monsters including the Leviathans would be after him. They inform him of the escape method Benny knows, though Benny admits that he doesn't know if it will work for an angel and convince him to reluctantly join them in their search for a way out. Later, in another flashback, Castiel is shown reaching out for Dean and the two losing their grip on each other.
In a flashback, Dean, Benny and Castiel come under attack by three monsters and kill them all, wondering where monsters go when killed in Purgatory. They argue about the fact that Castiel's presence is drawing monsters to them and that he may not be able to make it through the way out as he's not human or former human, but are forced to run when Castiel detects Leviathans nearby. Two Leviathans ambush them and Dean decapitates one while the other gets the drop on Castiel and nearly kills him before Benny decapitates her.
In flashbacks, Dean, Benny and Castiel make their way through Purgatory to the portal which Castiel is still not sure will let him out as he is not human. As they argue about the portal, it opens nearby in reaction to Dean's presence and Dean casts a spell to absorb Benny's soul into his arm. Dean and Castiel head for it, but come under attack by two Leviathans. They defeat and decapitate the Leviathans and head for the portal again which is closing. Dean gets through as it closes, but Castiel purposely lets go as he wants to remain behind in Purgatory. Dean remembers this as losing his grip on Castiel, but Castiel later shows him the truth.
In the present, Castiel is rescued from Purgatory by other angels under the command of Naomi though it costs several their lives.
When Sam needs to travel to Hell, rogue Reaper Ajay takes him to Purgatory as there is a portal to Hell there behind a boulder located between three trees that grow together. After telling him to meet him where he dropped Sam off in precisely 24 hours, Ajay leaves. After killing a vampire, Sam finds the portal and heads into Hell.
After rescuing Bobby Singer, the two return through the portal to find no sign of Ajay as he has been killed by Crowley. Trapped, they are attacked by three more vampires, but are saved by Benny who has returned to Purgatory through death to save Sam at Dean's request. Benny leads the two to the portal out and Sam absorbs Bobby's soul into his arm, but Benny, who never intended to return to Earth, stays behind to hold off more vampires. Sam escapes through the portal with Bobby's soul as Benny is torn apart.
Purgatory in Lore
- ↑ 1.0 1.1 6.20 The Man Who Would Be King
- ↑ 2.0 2.1 6.12 Like a Virgin
- ↑ 3.0 3.1 6.21 Let It Bleed
- ↑ 7.01 Meet the New Boss
- ↑ 8.01 We Need to Talk About Kevin
- ↑ 8.07 A Little Slice of Kevin
- ↑ 8.19 Taxi Driver
- ↑ 6.22 The Man Who Knew Too Much
- ↑ 7.01 Meet the New Boss
- ↑ 7.23 Survival of the Fittest
- ↑ 8.01 We Need to Talk About Kevin
- ↑ 12.0 12.1 8.07 A Little Slice of Kevin
- ↑ 13.0 13.1 13.2 8.19 Taxi Driver